The invention relates to a process for the low-temperature fractionation of air in a distillation column system that has at least one separating column, in which feed air is compressed in a main air compressor, compressed feed air is cooled down in a main heat exchanger, cooled-down feed air is introduced into the distillation column system and at least one product stream is drawn from the distillation column system, warmed up in the main heat exchanger and drawn off as a gaseous end product, wherein at least one process parameter is set by a basic controller, especially the closed-loop control of such a process, in particular during variable operation.
Processes and apparatuses for the low-temperature fractionation of air are known for example from Hausen/Linde, Tieftemperaturtechnik [cryogenics], 2nd edition 1985, Chapter 4 (pages 281 to 337).
The distillation column system of the invention may be formed as a one-column system for nitrogen-oxygen separation, as a two-column system (for example as a classic Linde double-column system), or as a three- or multi-column system. In addition to the columns for nitrogen-oxygen separation, it may have further apparatuses for obtaining high-purity products and/or other air components, in particular noble gases, for example argon production and/or krypton-xenon production.
The “main heat exchanger system” serves for cooling feed air in indirect heat exchange with return streams from the distillation column system. It may be formed by a single or a number of heat exchanger portions connected in parallel and/or in series, for example by one or more plate heat exchanger blocks.
Low-temperature air fractionation systems make high demands on the overall process control, both in terms of the type of system and in terms of the requirements with respect to capabilities under changing loads and optimizations of yield. They are characterized by an intensive intercoupling of the individual columns and apparatuses by heat and material balances. From a control engineering perspective, they represent a highly intercoupled multi-variable system. Moreover, the setpoint values of the variables to be controlled (analyses, temperatures, etc.) are dependent on the respective load case. On the other hand, for example, systems for producing gaseous products must quickly keep production in step with customer demand, and nevertheless at the same time ensure the highest possible product yield (in particular of oxygen and/or argon).
A “basic controller” controls a process parameter to a specified setpoint value. Such a “process parameter” is formed by a physical variable that has an influence on the fractionation process, for example by the pressure, the temperature or the throughflow at a specific point of the system or in a specific process step (PIC—pressure indication control, TIC—temperature indication control, FIC—flow indication control).
The “basic controller” may be a P controller (proportional), a PI controller (proportional integral), a PD controller (proportional derivative) or a PID controller (proportional integral derivative). Alternatively, two or more such controllers may be connected to one another as a cascade controller and be used as a basic controller. The entirety of the basic controllers is realized together with the necessary interlocking and logic circuits on a “control system”.
An “ALC control” (ALC=automatic load change) operates a level higher and specifies setpoint values for one or more basic controllers, preferably for the complete system, that is to say for all of the basic controllers. It is thereby possible to change automatically between the different load cases of a low-temperature air fractionation system. This technique is based on an interpolation between a number of load cases set and recorded in trial operation. In order to adopt a new load case, the target setpoint values of the individual basic controllers of the control system are precalculated and then adopted by means of a synchronized ramp, that is to say adjusted in small temporal increments within a specified time period.
The ALC control therefore specifies to the basic controllers a tested path to the load case that is to be achieved. As a result, a very high rate of adjustment is obtained. Any closed-loop control takes place in the basic control, for example by cascade controllers. Specifically, so-called trimming controllers on the control system are used, a basic controller setpoint value (mean value) that is precalculated by the ALC being corrected by a cascade circuit. The setpoint value of the cascade controller may likewise be specified by the ALC.
The various load cases of a low-temperature air fractionation system differ from one another in one or more of the following parameters:                amount of product of one or more product streams        ratio of amount of liquid product to amount of gas product        
The recording of the load cases for the ALC control is generally performed during the commissioning of the system over the entire operating range. In this case, the corresponding load cases are manually adopted and tested. These cases are stored in a mathematical model in the ALC; the various transitions between load cases can be subsequently tested.
An alternative to ALC controls are “MPC controllers” (MPC—model predictive control). This technology is widely used in the industry for controlling difficult and intercoupled multi-variable controlled systems. The basis is a mathematical model, which depicts the variation over time of controlled variables (CV) in response to changes of manipulated variables (MV). It is customary in control engineering to use simple linear models of the first order with dead time. Alternatively, more complicated, for example non-linear, models may also be used. The entire process is described by many such models in a matrix presentation. This process model is used for the closed-loop control, in that the behavior of the system in the future is simulated and finally the variation over time of the manipulated variables is calculated such that the system deviations are minimized and limit variables (LV) are maintained. An MPC controller allows account to be taken of the mutual interrelationships, and thereby makes particularly stable operation possible.
MPC controllers can control a low-temperature air fractionation system well in steady-state operation. Load changes mean for the MPC controller the specification of new target setpoint values for measurable production amounts, and the MPC then adjusts the entire process to the new load case. The course taken in the load change and the duration are not predictable; they are usually much slower than in the case of an ALC and often very unsmooth. There is no mechanism for specifying setpoint values load-dependently.
An ALC control allows rapid load changes and at the same time keeps the process much more stable than an MPC controller by simultaneous (synchronous) adjustment of all the relevant basic controllers under its control. On the other hand, however, the advantages of multi-variable control are not enjoyed.
MPC and ALC are both techniques of sophisticated process control that operate on the basis of setpoint values of the basic controllers under their control in order to adapt production and to control measured values (analyses, temperatures). They have so far been generally regarded as mutually exclusive control technologies.
Air fractionation systems with MPC controllers are known from EP 1542102 A1 and “Air Separation Control Technology”, David R. Vinson, Computers and Chemical Engineering, 2006.
The invention is based on the object of providing a process of the type mentioned at the beginning and a corresponding apparatus that make both particularly stable operation and rapid load changing possible.
This object is achieved by a process for the low-temperature fractionation of air in a distillation column system that has at least one separating column, in which                feed air is compressed in a main air compressor.        compressed feed air is cooled down in a main heat exchanger,        cooled-down feed air is introduced into the distillation column system and        at least one product stream is drawn from the distillation column system, warmed up in the main heat exchanger and drawn off as a gaseous end product,        wherein at least one process parameter is set by a basic controller, characterized in that        the control of the process parameter is performed by a combination of an ALC control and an MPC controller,        wherein the ALC control contains a set of measured values of the parameter that have been recorded during trial operation of the system and correspond to the various load cases and the transitions between these load cases, wherein also        the ALC control outputs a first target value to the MPC controller,        the MPC controller calculates from the first target value a setpoint value or a change in the setpoint value for a primary setpoint value output by the ALC control, and        the setpoint value determined by the MPC controller or a secondary setpoint value that is calculated from the primary setpoint value output by the ALC control and the change in the setpoint value is transferred to the basic controller.        